The present invention is directed to pressure sensitive hot melt adhesives used to form the hinge joint of a flexible covered book.
Flexible or soft covered books are generally formed by coating a layer of hot melt adhesive on the secured edge of a series of printed stocks or signatures to form a book block to which a paper backing or other suitable cover is applied to form the spine of the book. While this process produces the primary bonding of the book block to the cover, a secondary bond is often formed at the hinge or joint of the cover. This bond is produced by applying a small amount of adhesive to the outer sheet of each side of the book block immediately adjacent to the hinge area. This hinge joint formation is actually carried out in two separate steps with the adhesive being applied to the book block before the cover is affixed and the joint then being formed by application of pressure to the book cover at some point "downline" in the manufacturing operation. The purpose of this adhesive application is to conceal the binding adhesive and also to reinforce the binding and to prevent the book from falling apart relatively easily when grasped only by the flexible cover.
The adhesives used for this application must possess a variety of properties, specifically they must possess low viscosity (i.e., about 500 to 5000 cps at 350.degree. F.) for easy application, strong agressive tack at the point of formation of the hinge bond, good adhesion to difficult cover stocks and resistance to creep or cold flow both at room temperature and under elevated temperature conditions. Heretofore the adhesives used have been either the non-pressure sensitive hot melt adhesives used in the book binding operation or emulsion adhesives. Both types of adhesives suffer from the common deficiency of their relatively short open time i.e., the time during which a bond may be found. This lack of open time, creates a serious problem in the automated book binding process where interruptions, slow downs or even shut downs of the manufacturing line result in loss of agressive tack properties with the subsequent need to discard large quantities of partially completed products. Another problem resulting from the finite open time of the adhesives of the prior art is the high rejection rate due to poorly formed joints caused by the relatively high coating weights which must be applied in order to assure the presence of the needed residual agressive tack at the time the hinge joint is formed, particularly when slow machine speeds are involved. Additional rejects are created with finite open time products, when appropriate thin coating weights are used for high production speeds, but when machine slows down or stops because of some other interruption in the binding line the thin coating weight causes solidification prior to bond formation and therefore no bond can be formed.
It is therefore an object of the present invention to provide an adhesive for the hinge joint of a flexible book which has the cohesive strength, cold flow resistance, etc. of previous adhesives without being limited with respect to loss of agressive tack over time.
This and other objects will be apparent from the description that follows.